Wireless telecommunications systems continue to evolve. Examples of such systems are GSM (Global System for Mobile Communication) and UMTS (Universal Mobile Telecommunication System). Each such wireless communication system typically includes a radio access network (RAN). In UMTS, the RAN is called UTRAN (UMTS Terretrial RAN). A UTRAN includes one or more Radio Network Controllers (RNCs), each having control of one or more Node Bs, which are wireless terminals configured to communicatively couple to one or more UE terminals. The combination of an RNC and the Node Bs it controls is called a Radio Network System (RNS). A GSM RAN includes one or more base station controllers (BSCs), each controlling one or more base transceiver stations (BTSs). The combination of a BSC and the BTSs it controls is called a base station system (BSS).
Since the turn of this century, advancements in GSM, such as advanced frequency hopping, have greatly enhanced GSM voice capacity. Despite these advances, interference control is now largely influenced by cell site placement and antenna optimization. However, a relatively recent interference mitigation technique called Single Antenna Interference Cancellation (SAIC) shows promise. Historically, the most common method of reducing the impact of interference in a wireless system has been to use multiple antennas, often referred to as receive or antenna diversity. Conceptually, diversity provides multiple “diverse views” of the signal being transmitted in the presence of interference, so that the view which best overcomes the perturbations of the radio channel and interference is preferred for the receiver. Nevertheless, when receiving signals via two antennas, the hardware and software complexity increases, and hence the implementation cost is significantly higher. Therefore, today, multiple antennas are mainly used in base stations rather than in mobile terminals.
An alternative to “receive diversity” is interference cancellation using a single antenna at the handset. For example, SAIC may include joint demodulation, or may include newer low-complexity techniques involving a “virtual second antenna” which can be used to improve the robustness of the receiver in interference limited scenarios. Generally speaking, SAIC involves the class of algorithms enabling interference cancellation without exploiting a second receive antenna.
Many of the major wireless networks are asynchronous, meaning that any given base station does not attempt to align its transmitted signals with other base stations. The most powerful SAIC methods tend to be sensitive to the amount of overlap of the interfering signals with the desired signal. Typically, the best performance is achieved when the dominant interfering signal (burst) does not change characteristics throughout the desired signal's burst. One way to ensure this is to synchronize base stations and align all bursts to a common timing source, e.g. a global positioning system (GPS) clock.
As with any potential improvement in performance on either the uplink or downlink, it should be ensured that equal or better gains can be achieved on the other link. SAIC has primarily achieved a downlink gain. It is also desirable to have improvements for interference cancellation on the uplink, where multiple receive antennas are commonly used. For example, a multiple antenna interference cancellation technique known as Interference Rejection Combining (IRC) has been shown to provide network capacity gains of up to 50% in a synchronized network. The larger computational capabilities in base stations suggests that downlink receiver improvement can be matched by uplink performance using IRC. Hence, the downlink has usually been considered to be the limiting factor in GSM.
Statistical distribution of interference defines the performance of a communications link in capacity-limited conditions, where the carrier-to-noise ratio is negligible compared to carrier-to-interference ratio. Network synchronization can guarantee good statistical distribution of interferers for SAIC/IRC receivers. The Synchronization System Solution achieves a timeslot (TS) synchronization in all cells of the network by means of a common GPS clock signal and local measurement unit (LMU), which is required in every base station. Currently, no simpler way has been implemented to synchronize the network.
In contrast, when a network is non-synchronized, time-variant interference distribution is based on several network parameters such as frequency band allocation, frequency reuse, frequency load, cell size, DTX, frequency hopping parameters, and so on. Therefore, interfering signals are randomly overlapping with the user signal, and since there are many factors contributing to the overall achievable interference-limited performance in a non-synchronized network, the effect and gain of interference cancellation may not always be visible.